Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be an fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
OLED devices are generally (but not always) intended to emit light through at least one of the electrodes, and one or more transparent electrodes may be useful in an organic opto-electronic devices. For example, a transparent electrode material, such as indium tin oxide (ITO), may be used as the bottom electrode. A transparent top electrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, may also be used. For a device intended to emit light only through the bottom electrode, the top electrode does not need to be transparent, and may be comprised of a thick and reflective metal layer having a high electrical conductivity. Similarly, for a device intended to emit light only through the top electrode, the bottom electrode may be opaque and/or reflective. Where an electrode does not need to be transparent, using a thicker layer may provide better conductivity, and using a reflective electrode may increase the amount of light emitted through the other electrode, by reflecting light back towards the transparent electrode. Fully transparent devices may also be fabricated, where both electrodes are transparent. Side emitting OLEDs may also be fabricated, and one or both electrodes may be opaque or reflective in such devices.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As the information revolution taking place all round us, there is a tremendous explosion in the amount of data being generated, transmitted, and in need to be analyzed. An information display remains the most efficient way that a person can interact with these data. The display device of choice is a flat-panel display, but the current liquid crystal display (LCD) technology in use by most flat-panel displays is limited in its ability to meet the increasing demands. A new display technology, however, offers considerable promise for overcoming the limitations of the LCD technology. The new technology is based on the application of organic light-emitting devices (OLEDs), which make use of thin film materials that emit light when excited by an electric current.
The typical OLED consists of a multi-layer sandwich of a layer of indium tin oxide (ITO) (tITO˜100 nm, nITO˜1.8-2.0), one or more organic layers (torg˜0.1 nm, norg=1.6-1.8 or higher), and a cathode (e.g. Mg:Ag, LiF:Al, or Li:Al), where t refers to the layer thickness and n refers to the layer index of refraction. These layers are most often deposited on tip of a planar glass substrate (e.g. soda lime glass, n=1.51), with plastic substrates (n˜1.5-1.6) being the most popular substitute. These commonly used substrates whose index of refraction is lower than that of the emitting material will be referred to as the standard substrates herein. For simplicity, the discussion herein will be based on a PVK/Alq3 bi-layer device. However, those skilled in the art will readily understand that the discussion and analysis that follows can readily be extended to single layer devices or other more complicated device structures.
One factor considered in evaluating a display system is the efficiency of conversion of input power to emitted light. In OLED displays, one factor used in determining this system efficiency is the external coupling efficiency (ηext) with which internally generated light is coupled out of the device. In order to meet expected demands of future display systems, there is a need to improve the coupling efficiency of OLEDs.
In a conventional planar OLED as described above, a large amount of light is waveguided in the substrate, ITO, and organic layers, and emitted through the edge or lost due to absorption. See, N. C. Greenham, R. H. Friend, and D. D. C. Bradley, “Angular dependence of the emission from a conjugated polymer light-emitting diode: implications for efficiency calculations”, Adv. Mater., vol. 6, pp. 491-494, 1994. Consequently, various schemes have been proposed to shape the substrate thereby destroying the substrate waveguide and allowing more light to be emitted externally. See, G. Gu, D. Z. Garbuzov, P. E. Burrows, S. Venkatesh, and S. R. Forrest, “High-external-quantum-efficiency organic light-emitting devices”, Opt. Lett., vol. 22, pp. 396-398, 1997, T. Yamasaki et al., “Organic Light Emitting Device With an Ordered Monolayer of Silica Microspheres as a Scattering Medium,” App. Phys. Lett., vol. 76, pp. 1243-1245 (2000); C. F. Madigan, M.-H. Lu, and J. C. Sturm, “Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification”, App. Phys. Lett., vol. 76, pp. 1650-1652, 2000, which are incorporated by reference in their entireties. Of these schemes, those that pattern the substrate on the backside (the non-device side) are more compatible with display manufacturing processes. Prior to the present invention, it is believed that these techniques have only been demonstrated on substrates whose index of refraction is less than that of the emitting material.